|
apple-green; (c) Phrase—translucent and leek green; (d) Plasma—some shades of green, often of a dark green colour—stalactitic or concretionary forms as linings of cavities—light-coloured, white, grey or brown or blue form of quartz with a waxy lustre. Chalcedony is cut cabochon.
(ii) Heliotrope or bloodstone.—A dark green chalcedony containing scattered spots of red jasper—used for engraved objects a,nd signet rings.
(iii) Agate
It is a very popular and crypto-crystalline variety of quartz and occurs in geodes of volcanic and sedimentary rocks and as pebbles. The colours are dispersed in parallel bands, which are more or less wavy. Colour—white, brown, red, blue, etc. Agate may be pseudomorphous after wood. Idar-Oberstein is the place in Germany for cutting and polishing agate. Agate is also artificially coloured.
(iv) Onyx
Similar to agate, with coloured bands straight and parallel. It is used for cameos.
(v) Jasper
Opaque, red, yellow, brown, dark green or greyish blue. When the colours occur in broad bands or stripes, it is called riband jasper.
(vi) Flint
Transparent to opaque, grey, smoky brown or black, used by early man to make weapons and also fire. It is found in chalk and has a white coating.
Occurrence
Bock crystal is a very common gem mineral found in veins in metamorphic rocks and in pegmatite dykes. It is obtained from Japan, Brazil, Madagascar, Switzerland, New York and Arkansas. The main supply for optical purposes
143
|
||
|
|
||